Double-winding generator with alternate poles



March 6, 1934. c. M. LAFFOON ET Al.

DOUBLE WNDING GENERATOR WITH ALTERNATE POLES Filed Dec 50, 1929 llB ATT NE Patented Mar. 6, 1934 DOUBLE-WINDING GENERATOR WITH AL- TEBNATE POLES Carthrae M. Laoon, Irwin, Lee A. Kilgore, Forest Hills, and Rolla E. Powers, Verona, Pa., assignors to Westinghouse Electric and Manufacturing Company, a corporation of Pennsylvania Application December 30, 1929, Serial No. 417,306

7 Claims.

Our invention relates to double-winding generators which are used to supply power to two di'er'ent bus sections.

The double-winding generator is a recent development, in the design of large generators, which reduces the short-circuit currents and the load currents to be handled by generator reactors and breakers. Since high currents constitute one limiting factor in the application of very large 10 units, double-Winding Igenerators will, undoubtedly, have an increasingly large consideration in the design of future central stations.

In an article by D. D. Chase and H. C. Forbes in the Electrical World of December 15, 1928, page 1183, the advantages of double-winding generators in central-station design were discussed, and a particular type of generator having the two windings disposed in alternate slots was advocated. This discussion and advocacy were continued in an article by T. F. Barton in the General Electric Review for June, 1929, page 302.

The articles just mentioned entirely overlook the effect of saturation of the single tooth separating the corresponding inductors of the two groups of windings in the alternate-slot-winding generator during periods when fault currents of many times the normal rated current are ilowing through the machine so that the currentow in one of the windings is reversed. Under these conditions, which are the real conditions for which the double-winding generator is primarily designed, the single tooth becomes saturated and the effect is to appreciably reduce the through reactance and to increase the slot-leakage-iiux coupling between the two windings.

This effect of saturation, on the so-called through reactance, was discussed in a paper by C. M. Laioon presented before a meeting of the Empire Gas and Electric Association at Buffalo, N. Y., in April, 1929; and again in a paper by C. M. Laffoon presented before a meeting of the Association of the Edison Electric Illuminating Companies at Philadelphia on May 17, 1929, copies of which were printed or reproduced and mailed to all of the Edison Companies in the United States in May, 1929. These effects were again discussed in an article by L. A. Kilgore and R. E. Powers in the Electric Journal for October, 1929, page 480. The limiting effects of saturation in the alternate-slot double-winding generator were also discussed in a paper by P. L. Alger, E. H. Freiburghouse and D. D. Chase, presented at a meeting of the American Institute of Electrical Engineers at Chicago, Illinois, on December 3, 1929.

The articles and papers hereinabove mentioned are referred to for the purpose of affording an explanatory and descriptive background and setting for the present invention, and to avoid unnecessary repetition in this specication.

The object of our present invention is to provide a specially chorded double winding of the type utilizing winding groups in alternate poles connected in series, or, in general, utilizing such disposition of the two windings that a large majority of the conductors or inductors of each Winding are separated from a normally substantially in-phase conductor of the other winding, by a distance greater than one tooth-pitch. It will be noted that the alternate-pole type of winding has been unjustly condemned, heretofore, as being unsuitable for use in systems where double-winding generators are used, because of theoretical considerations which erroneously led to the conclusion that any inequality in the loads fed from the two circuits would produce severe magnetic unbalance, resulting in mechanical vibration and impossible operating conditions on account of high losses. Our theoretical analysis and actual tests on machines, as mentioned in our papers and article above cited, have indicated, on the contrary, that, with proper chording, the alternate-pole design will not involve excessive losses when the degree of unbalance in load does not exceed ten percent, which is in accordance with the ordinary central-station practice; and it has the very decided advantage of a through reactance which is so high that, in general, external generator reactors and bus reactors could be entirely omitted, thus saving a very considerable item of expense, which was not possible in the case of the generator having two windings in alternate slots.

In the accompanying drawing,

Figure 1 is a diagrammatic view of apparatus and circuits embodying our invention in a central station, or a portion of a central station;

Fig. 2 is a diagrammatic development of the winding, showing the disposition of the threephase groups of each of the two parallel winding. circuits in the slots of a machine;

Fig. 3 is a wiring diagram showing the connections of one phase of each winding circuit;

Figs. 4 and 5 are curve diagrams hereinafter referred to; and

` Fig. 6 is a schematic diagram of an equivalent circuit.

In the portion of a central station shown in Fig. 1, three bus sections B1, B2, Bs; B1', B2', Ba'; and B1, 13;, 33"; are provided, the same being normally separated by means of circuit breakers and disconnecting switches '7. Each bus section is provided with a plurality of feeders 9. The bus sections are illustrated as being supplied with energy from two double-winding synchronous generators 11 and 12 in such manner that the two windings of each generator are connected to two different bus sections, so that, except for such connection as may be afforded by the loads at the ends of the feeders 9, the bus sections are connected only by the magnetic coupling between the two circuits.

As shown in the case of the generator 1l, the terminals T4, T5, Ts of one of the three-phase windings are connected in star, and the corresponding terminals T4', T5', Ta of the other winding are also connected in star, leaving three terminals Ti, T2, T3 of one winding and the terminals Ti', Ta', T3 of the other winding, to be connected to the buses Bi, Bz, Ba and B1', B2', B3', respectively. The application of a winding of this type is not limited to the star connection but may be used equally well with windings connected in delta. Suitable breakers and disconnecting switches, indicated schematically at 14, are utilized for effecting the connection between the machines and the bus sections. In the system shown in Fig. l, no reactors are connected in the generator leads in series with the breakers 14, because, with our type of winding, such reactors are not, in general, required. However, it will be understood that such reactors may be added, in accordance with the former practice, but considerably reduced in size, as compared with the reactors heretofore used.

In Fig. 2, the three-phase windings of one winding group are indicated by solid triangles, circles and squares, respectively, whereas the three-phase windings of the other winding group are indicated by open triangles, circles an! squares. A wiring diagram showing one of the phases of each winding group is indicated in Fig. 3. It will be noted that the windings under alternate poles are connected in series (by which expression we mean to include two-pole as well as four-pole machines); hence the name, alternate-pole winding.

As indicative of the fact that the losses encountered in double-winding alternate-pole machines of the type just described are not excessive, we have indicated, in Fig. 4, the results of tests showing the relation between the total temperature rise of the eld winding, in comparison with the load, for different degrees of loading and unbalanced conditions on the two windings. We have found that, if the unbalance in the load on the two windings is such that one winding carries forty-five percent of the entire rated capacity of the machine, whereas the other winding carries fifty-five percent of the same, the increase in heating over normal full-load operating conditions is only iive degrees, which is quite acceptable. In metropolitan systems, having facilities for shifting feeders from one bus section to another, it is quite possible to keep the operating conditions within these extreme values, even under peak-load conditions.

It will be noted, from Figs. 2 and 3, that the winding is chorded to such extent that the winding pitch is two-thirds of the pole pitch. This is the chording which gives approximately the maximum through reactance in the machine, although it also gives higher losses, at unbalancedload con lltions, than smaller amounts of chording. In lJome instances, that is, where the facilities for maintaining balanced-load conditions are not suillclently good, chordings of fivesixths pitch, or approximately eighty-five percent pitch, may be desirable, even though the through reactance is reduced thereby to about four-sevenths of its value with two-thirds pitch, thus probably necessitating the utilization of small external reactors to make up for the deciency in the reactance.

The effect of grouping together all the conductors of each phase belt of each pole is to make the saturation factor, which we designate by the letter K, nearly equal to unity. This will be understood by reference to the equivalent-circuit diagram in Fig. 6 which indicates the equivalent reactances between the two buses B, B', and the generator G.

If the mutual reactance of the two generator windings is XM, resulting from the ilux linking the two stator windings but not the rotor winding, and if the leakage component is Xi, resulting from the flux linking one winding only, the reactance to the flow of the generator current during normal operating conditions is (XM-i-Xi) for the windings in parallel. The reactance to the ow of through current, which we call the through reactance XT is 4X1, or four times the leakage component of the reactance with the windings in parallel, the series reactance being four times the parallel reactance. In the normal equivalent-circuit diagram, the saturation factor K is ignored, being assumed to be equal to unity. In our Fig. 6, we have indicated this factor as being present, and in Fig. 5, we have shown how this factor varies for different through fault currents, expressed in terms of the full-load rated current of the machine, including both windings. Thus, for a single-winding generator, the saturation factor is nearly unity, being .9 when the short-circuit current is nine times the full-load current. In our alternate-pole machine, with two-thirds pitch, the saturation factor is not much smaller, being .'18 when the fault current passing through the machine is nine times the full-load current. In `the alternate-slot machine, however, it will be seen that the saturation factor K drops to .15, under these conditions. As will be evident from Fig. 5, the above-mentioned values of the saturation factor K become approximately .94, .84 and .35, respectively, for the three types ot windings, when the through current is only three times the rated full-load current of the generator.

In Fig. 5, the full-load current is taken to be the rated current of one of the windings, or ifty percent of the rated capacity of the entire machine. It is our practice to divide the actual currents owing in the two windings, as, for instance, when a fault current is flowing through the machine ll from the bus B1' to a fault at X on one of the feeders 9 of the bus B1, two components, a through component and a generator component. The through currents are equal in magnitude and opposite in direction, and the generator currents are equal in magnitude and in the same direction, that is, both flowing out of the machine. Deflned in this way, the through current is the average of the two winding currents, assumed to be flowing in opposite directions, as for a short circuit. The generator current in each winding is the average of the two winding currents assumed to be flowing in the same direction.

Our invention is particularly applicable to large two and four-pole synchronous generators having two or more windings in parallel. While we have explained our invention in connection with a single generator design, and in connection with a single central-station application, it will be understood that various changes and departures may be made, as will be evident to those vskilled in the art, without departing from the essential spirit of our invention or sacrilcing the essential benilts derived therefrom. We desire, therefore, that the appended claims shall be accorded the broadest 'construction consistent with their language and the prior art.

We claim as our invention:

1. A double-winding generator in which the phase groups of alternate poles are brought out to different terminals, characterized by the fact that the coils are chorded to about pitch to separate the two windings, and the coil-sides of each phase are arranged inl groups to cause the saturation factor, for through fault currents of three times the full-load current, to be greater than 0.8.

2. A polyphase double-winding dynamo-electric machine in which the phase groups of alternate poles are brought out to different terminals, characterized by the fact that the winding has a fractional pitch less than about 85% of the pole pitch.

3. A double-winding alternating-current dynamo-electric machine in which the phase groups of alternate poles are brought out to dii'- ferent terminals', characterized by the fact that the winding has a fractional pitch less than about 85% of the pole pitch.

4. A polyphase double-winding synchronous dynamo-electric machine in which the phase groups of alternate poles are brought out to different terminals, characterized by the fact that the winding is chorded to about pitch.

5. An electrical system comprising a plurality of bus-sections and a plurality of generators including a two-winding generator having its two windings connected to two different bus-sections, at least some of the bus-sections being connected to windings of at least two different generators, characterized by the fact that said twowinding generator has a winding in which the phase groups of alternate poles are brought out to different terminals, and characterized further by the fact that the winding has a fractional pitch less than about 85% of the pole pitch.

6. An electrical system comprising a plurality of bus-sections and a plurality of polyphase generators including a two-winding generator having its two windings connected to two different bus-sections, at least some of the bus-sections being connected to windings of at least two different generators characterized by the fact that said two-winding generator has a winding in which the phase groups of alternate poles are brought out to different terminals, and characterized further by the fact that the winding has a fractional pitch less than about 85% ofl the pole pitch.

7. An electrical system comprising a plurality of bus-sections and a plurality of polyphase generators including a two-winding generator having its two windings connected to two different bus-sections, at least some of the bus-sections being connected to windings of at least two different generators, characterized by the fact that said two-winding generator has a vwinding in which the phase groups of alternate poles are brought out to different terminals and characterized further by the fact that the winding is chorded to about 173 pitch.

CARTHRAE M. LAFFOON. LEE A. KILGORE. ROLLA E. POWERS. 

